A digitizer of simple construction is of the wedge and strip type. A digitizer of this type comprises a digitizing tablet and cursor or stylus movable thereon, wherein an electrical field coupling is utilized to ascertain the position of the cursor or stylus on the digitizer tablet. The coordinate system is typically the x-y coordinate system.
The cursor or stylus comprises a housing movable on the digitizer tablet, the housing containing a coupling electrode. In one embodiment an input signal is applied to the coupling electrode of the cursor or stylus. The coupling electrode field electrically couples to wedge (sawtooth) and strip patterned electrodes embedded in the digitizer tablet. One of the wedge or strip patterned electrodes is used to ascertain the x-axis position of the cursor and the other is used to ascertain the y-axis position of the cursor. The wedge and strip patterned electrodes are conveniently provided in a coplanar arrangement on a printed circuit board and covered with a layer of dielectric material to provide a smooth tablet surface. In the embodiments where the coupling electrode of the cursor or stylus is driven, the signals induced in the patterned electrodes are processed electronically to ascertain the position of the cursor relative to the tablet.
In accordance with the wedge-and-strip type of digitizer, disclosed in U.S. Pat. No. 4,705,919, assigned to the assignee of the present application and the disclosure of which is specifically incorporated herein by reference, the coupling electrode outputs the induced signal and the patterned electrodes are driven. The digitizer tablet has three patterned electrodes which produce an x-axis (X) signal, a y-axis (Y) signal and a balance (B) signal. The X electrode comprises a plurality of conductive rectangular strips disposed vertically and arranged at equal intervals and in order of increasing width from one side to the other side of the tablet, e.g., increasing in width from the left to the right side of the tablet. The Y electrode comprises a plurality of uniform conductive wedges or sawtooths also disposed vertically and arranged at equal intervals with the Y wedges interleaved with the X strips. Movement of the cursor in the x-axis direction has a negligible effect on the cursor coupling with the Y electrode and movement of the cursor in the y-axis direction has a negligible effect on the cursor coupling with the X electrode. The balance (hereinafter "B") electrode substantially covers the area of th digitizer tablet not covered by the X and Y electrodes, except for interelectrode spacing therebetween.
The coupling electrode located in the cursor housing and movable on the surface of the digitizing tablet surface is connected to an oscillator whose frequency typically is in the range of 10 kHz to 1 MHz. The coupling electrode induces electrical signals in the X, Y and B electrodes.
The surface area of the X electrode coupled with the coupling electrode increases as the cursor or stylus is moved or positioned toward the one side of the tablet where the strips are wider, and hence more electrical signal is induced in the X electrode. Similarly, the surface area of the Y electrode coupled with the coupling electrode increases as the cursor or stylus is moved to a region on the tablet where the wedges are wider, and hence more electrical signal is induced in the Y electrode. The X, Y and B electrodes completely cover the position coordinate determination area of the tablet except for narrow spacing between the electrodes. It is therefore expected that the sum of the signals on the X, Y and B electrodes will be substantially independent of cursor (stylus) position. This sum signal depends primarily upon the amplitude of the oscillator signal, the proximity of the coupling electrode to the X, Y and B electrodes, and the dielectric constant of the material between the coupling electrode and the X, Y and B electrodes.
Based upon principles of mathematics and electronics, the x coordinate of the cursor is proportional to the X electrode signal divided by the sum of the X, Y, and B electrode signals. Similarly the y coordinate of the cursor is expected to be proportional to the Y electrode signal divided by the sum of the X, Y and B electrode signals. In theory, this ratiometric technique should enable accurate measurement of the cursor (stylus) position and be independent of the proximity of the cursor (stylus) to the digitizer tablet and the dielectric therebetween.
Alternatively, the functions of the coupling electrode and the patterned electrodes can be reversed, i.e. input signals are successively applied to the X, Y and B electrodes, and induced signals are outputed from the coupling electrode in response thereto.
In the preferred embodiment of the tablet disclosed in U.S. Pat. No. 4,705,919, a coupling electrode located in a cursor housing and movable over the surface of the digitizer tablet is coupled to one or more of the X, Y and B electrodes, which are driven in accordance with a predetermined pattern. Alternatively, the coupling electrode can be incorporated in a stylus. (Hereinafter the term "cursor" will be generally used with the understanding a stylus can be used in place of a cursor in the invention.) The signals capacitively induced in the coupling electrode are outputed to the processing circuitry, which determines the coordinate position of a point on the cursor therefrom with compensation for resolution distortion.
The surface area of the X electrode coupled with the cursor increases as the cursor is moved or positioned toward one side of the tablet where the strips are wider and accordingly when the X electrode is driven, the amount of electrical signal coupled to the cursor increases as the cursor moves in the direction of increasing strip width of the X electrode. Similarly, the surface area of the Y electrode coupled with the cursor increases as the cursor is moved to a region of the tablet where the wedges are wider, and accordingly when the Y electrode is driven, the amount of electrical signal coupled to the cursor increases as the cursor moves in the direction of increasing wedge width of the Y electrode.
Three separate driving circuits respectively apply square wave signals to the X, Y and B electrodes and coupling of these signals to the coupling electrode of the cursor is sampled in accordance with a predetermined sampling cycle. Each output signal is amplified, rectified, integrated and digitized to produce a digital signal representing the amount of capacitive coupling induced in the cursor by the one or more driven electrodes. A microprocessor performs mathematical operations on the digital signals in accordance with mathematical relationships for wedge and strip configurations to produce data X.sub.in and Y.sub.in representative of the x and y coordinates of the cursor position on the digitizer surface. In particular, EQU X.sub.in =2C.sub.X /(C.sub.X +C.sub.Y +C.sub.B) (1) EQU Y.sub.in =2C.sub.Y /(C.sub.X +C.sub.Y +C.sub.B) (2)
where C.sub.X is the digital value corresponding to the signal induced in the X electrode, C.sub.Y is the digital value corresponding to the signal induced in the Y electrode and C.sub.B is the digitial value corresponding to the signal induced in the balance electrode. The geometry of the patterned electrodes is such that O&lt;X.sub.in &lt;1 and O&lt;Y.sub.in &lt;1. The digital value representing signal output from the cursor or stylus when the X, Y and B electrodes are driven simultaneously, i.e. C.sub.X +C.sub.Y +C.sub.B, will hereinafter be referred to as C.sub.sum.
The C.sub.X, C.sub.Y and C.sub.sum values are obtained by adjusting the raw readings to compensate for the noise and the offset, as fully disclosed in U.S. patent application Ser. No. 28,499, assigned to the assignee of the present application and entitled METHOD AND APPARATUS FOR CALIBRATION OF A CHARGE RATIO DIGITIZER, in accordance with the following equations: EQU C.sub.X =A.sub.X -B.sub.X ( 3) EQU C.sub.Y =A.sub.Y -B.sub.Y ( 4) EQU C.sub.sum =A.sub.sum -B.sub.sum ( 5)
where A.sub.X, A.sub.Y and A.sub.sum are respectively the digital values (i.e. raw readings) obtained in response to driving of the X electrode alone, the Y electrode alone and the X, Y and B electrodes together when the stylus or cursor is enabled, and B.sub.X, B.sub.Y and B.sub.sum are respectively the digital values (i.e. background readings) obtained in response to driving of the X electrode alone, the Y electrode alone and the X, Y and B electrodes together when the stylus or cursor is disabled.
To compensate for the noise and the offset, a digitizer construction is provided wherein readings B.sub.X, B.sub.Y and B.sub.sum are taken when the stylus or cursor is disabled. Each background reading is the sum of the digital value corresponding to the environmental and inherent noise and the digital value corresponding to the offset. This summed background digital value is stored in memory and subsequently subtracted from the readings acquired during digitization, i.e. when the stylus or cursor is enabled.
The microprocessor also operates upon the acquired data using correction factors to compensate for resolution distortion. Because of the inherent distortions discussed above, the x and y coordinates cannot be determined simple from the values 2C.sub.X /C.sub.sum and 2C.sub.Y /C.sub.sum. The x and y coordinates can be determined only after a calibration has been performed to compensate for resolution distortion. This calibration is performed by processing the signal output from a small conductive plate, i.e. the so-called "phantom cursor" built into the digitizer tablet.
The patterned electrodes of the prior art charge ratio tablet form an active surface area useable to determine the positional coordinates of a stylus or cursor coupled to the tablet. However, this prior art tablet is disadvantageous in that no menu means are incorporated in the tablet for enabling function or mode selection using the cursor or stylus.